The largest known solitary hydroid, Branchiocerianthus imperator (Allman, 1885), was first collected on June 17, 1875, with a trawl-net, off Boso Peninsula on the Pacific coast of Japan, during the Challenger Expedition; it was described as Monocaulus imperator Allman, 1885. On June 21, 1985, 110 years after its discovery, a living specimen was observed for the first time, using the submersible "Shinkai 2000". The species is a suspension feeder and a carnivore: a symbiotic shrimp was found associated with it. Recently the species has been recognized from photographs taken in deep water of the N.E. Atlantic by the French IFREMER/CEA Expedition, apparently the first Atlantic record.